The early Ediacaran lower Wilpena Group of the Adelaide Superbasin, South Australia: a review of its stratigraphy and bounding ‘disconformity sequences’

The Ediacaran Period is represented throughout the Adelaide Superbasin of South Australia by two upward-coarsening sedimentary cycles that, together, constitute the Wilpena Group. The stratigraphic nomenclature applied to these two cycles, however, has not been consistently applied in accordance with the national code. The clastic components of the lower cycle coarsen upwards from an initial transgressive sequence comprising a basal dolostone and transitional zone of interbedded dolostones and shales through to shales and fine siltstones deposited under fully submerged conditions. These are followed by a regressive sequence of shallowing intertidal deltaic medium- to coarse-grained sandstones and cyclically interbedded fine units that, together, intertongue laterally with intertidal shales and siltstones that grade to sandstones before being capped by a veneer of terrestrial deposits dominated by coarse-grained sandstones and microconglomerates. Although deposited upon a locally erosive disconformity surface, these coarse-grained terrestrial sediments represent the peak of the regression and their upper boundary, the maximum regressive surface, coincides with the flooding surface marking the onset of the transgression that initiates the upper Wilpena cycle. This thin terrestrial sequence, bounded by a basal disconformity and capped by the maximum regressive surface, constitutes a ‘disconformity sequence’ at the sequence boundary separating the lower and upper cycles of the Wilpena Group. In a similar vein, a sequence of arenaceous fluvial deposits filling an incised valley lying immediately beneath the lower cycle represents a comparable ‘disconformity sequence’ at the top of the underlying Umberatana Group. The stratigraphic complexities within the lower Wilpena Group cycle (the transgressive/regressive Brachina sequence situated between the Marinoan Glaciation and just below the Acraman Asteroid Impact throughout the Adelaide Superbasin) are reviewed. The stratigraphic nomenclature historically applied to the Brachina sequence is reviewed, and the term <i>Brachina Subgroup</i> recommended for ratification. Terrestrial deposits generated at the peak of the Brachina Subgroup regression are analysed and defined in a sequence stratigraphic sense as a ‘disconformity sequence’. A similar ‘disconformity sequence’ lies immediately beneath the Brachina Subgroup.